Increasing concern about the benzene concentration in gasolines has prompted interest in changing refinery operations by shifting feeds between refinery units to minimize benzene production. By way of example only, the benzene in high benzene content feeds sent to reforming could be separated from the naphthas and routed elsewhere as the benzene passes through the reformer essentially unchanged while the naphthas are converted to higher octane products. It would be more efficient and desirable to remove the benzene from the reformer feed and route it to another process which would convert the benzene to a more agreeable blending component.
The isomerization unit would be suitable because the isomerization unit saturates aromatics to form cycloparaffins which are a preferable gasoline blending component. However, the presence of high levels of C.sub.6 hydrocarbons has been found to suppress the isomerization of lower molecular weight hydrocarbons, specifically, the conversion of C.sub.4 normal paraffin to isoparaffin. Thus, for purposes of efficiency, processes for the isomerization of C.sub.5 and C.sub.6 are separated from processes for the isomerization of the C.sub.4 hydrocarbons. In isomerization of C.sub.5 to C.sub.6 hydrocarbons low temperatures (ranging from 110.degree. to 180.degree. C.) favor reaction equilibrium toward higher isoparaffin to total paraffin ratios and avoid cracking. By comparison, in isomerization of butanes, relatively higher temperatures (greater than 150.degree. C.) favor the approach to reaction equilibrium toward higher isobutane to normal butane ratios. Additionally, while the presence of hydrogen is used in isomerization of C.sub.5 to C.sub.6 hydrocarbons to inhibit side reactions, high concentrations of hydrogen tend to inhibit isomerization of butanes.
In R. A. Meyers, Handbook of Petroleum Refining Processes, pp. 5-47 to 5-59 (1986) the UOP Penex process is described as a single-pass isomerization of C.sub.5 and C.sub.6 hydrocarbons in the presence of hydrogen in a two-stage reactor system using a low temperature noble metal fixed-bed catalyst which works in the presence of a promoter to produce a gasoline blending fraction of about 85 Research Octane Number. A high conversion at a low temperature, i.e. 230.degree. F. to 300.degree. F., is possible. Most of the i-pentane and i-hexanes of isomerization having a high RON are blended into gasoline. A UOP Butamer process is also described in Meyers at pp. 5-39 to 5-46. A fixed-bed vapor phase butane isomerization process is described in which a feedstream rich in C.sub.4 hydrocarbons is dried and charged to a one or two isomerization reactor system. The isomerization occurs at a temperature slightly higher than that used in the C.sub.5 to C.sub.6 isomerization, in the presence of a minor amount of hydrogen. The process converts butanes to isobutanes which are used to make high octane alkylate.
Butane isomerization in the presence of C.sub.5 and C.sub.6 hydrocarbons in a one or a two-reactor system has been proposed in U.S. Pat. No. 4,877,919 and this process is described as providing the advantages of reduced capital and operating costs associated with separate processing units for butane isomerization and C.sub.5 and C.sub.6 isomerization while achieving a relatively high across-the-board conversion of the C.sub.4 to C.sub.6 hydrocarbons. The benefits of using two reactors is described whereby the first reactor operates at higher temperatures to favor butane isomerization and the second reactor operates at a lower temperature to increase the C.sub.5 to C.sub.6 isoparaffin to total paraffin ratio without reversing isobutane yield. The isomerate product is then transferred to a stabilizer to remove the cracked hydrocarbons containing C.sub.3 hydrocarbons and lighter hydrocarbons and excess hydrogen. The patent teaches separating hydrogen from the product stream and recycling it to the C.sub.4 to C.sub.6 isomerization reaction. U.S. Pat. No. 4,877,919 demonstrates the benefit of isomerization for C.sub.4, C.sub.5 and C.sub.6 normal paraffins. However, there is no discussion of the detrimental effect of C.sub.6 cyclics on isobutene yield. C.sub.6 cyclics include benzene, cyclohexane, and methyl cyclopentane.
Although it would be advantageous to process C.sub.4, C.sub.5 and C.sub.6 hydrocarbons together through the isomerization reactor, the impact of large amounts (i.e. more than 20%) of C.sub.6 hydrocarbons, particularly the C.sub.6 cyclics, on the conversion of C.sub.4 hydrocarbons detracts from the efficiency of the process. A process for isomerizing C.sub.4 and C.sub.5 to C.sub.6 hydrocarbons separately in two reaction zones without the disadvantages of having entirely separate units would be most practical.